Dengue is an emerging mosquito-borne viral infection of humans. Infected people develop potent and long lasting antibodies (Abs) that neutralize the homologous dengue virus (DENV) serotype. Paradoxically, Abs have also been implicated in enhanced viral replication and more severe disease. Despite the evidence that Abs are important in dengue pathogenesis, we are just beginning to learn about the binding and functional properties of the human Ab response to DENV. The humoral immune response to a pathogen is derived from long-lived plasma cells (LLPCs) that contribute to circulating Abs and a memory B cell (MBC) pool that is activated upon re-exposure to the pathogen. While it is known that DENV infection stimulates robust LLPC and MBC responses, specific properties of Abs derived from these two compartments have not been characterized in detail. Based on recent findings from our group and others, we propose to test the hypothesis that new epitopes created by close packing of envelope (E) protein molecules on the viral surface are the main target of the human DENV serotype-specific neutralizing Ab response. People exposed to primary DENV infections develop long-term serotype specific neutralizing Ab responses whereas secondary infections result in serotype cross neutralizing responses. We propose that second infections preferentially activate somatically mutated MBCs expressing Abs that bind with high affinity to 2 or more serotypes and are capable of cross neutralizing serotypes. We propose three specific aims to characterize neutralizing antibodies derived from MBC and LLPCs after primary infections (Aims 1 and 2) and secondary infections (Aim 3). The impact of this work will be far ranging as it will redirect a field that has mainly focused on B cell epitope on subunits of E protein to consider new structural features and epitopes created following viral assembly. These studies proposed here are directly relevant to developing simple assays to predict the performance of the leading dengue vaccine candidates and also for developing the next generation of safe and effective dengue vaccines. .